The present invention relates to retroreflective detectors which are arranged to emit a beam of light from a transmitter, reflect the light beam by a retroreflector and receive the reflected beam of light by a receiver. In particular, the present invention is directed to an improvement for preventing false signalling operation in response to the presence of objects.
As a detector for detecting persons or the like, Japanese Patent Laid-open Publication No. H8-265130 (JP-A-265130/1996) discloses a retroreflective detector. A detector of this type comprises a detection unit housing a transmitter and a receiver, and a retroreflector positioned opposite to the detection unit with a prescribed distance therebetween. This retroreflector comprises a prism called corner cube reflector for reflecting a beam of light emitted from the transmitter. The retroreflector functions to reflect (retroreflect) the incident light emitted from the transmitter in the direction opposite to the incident direction.
Where no object is present in the space between the detection unit and the retroreflector, a light beam (e.g. infrared ray) emitted from the transmitter is reflected by the retroreflector and then the reflected light is received by the receiver. On the other hand, where an object (e.g. a person) is present in or passes through the space between the detection unit and the retroreflector, the object interrupts the light beam emitted from the transmitter, causing the intensity of light received by the receiver to change. Hence, the presence or passage of an object is detected by evaluating changes of the intensity of the reflected light beam which is received by the receiver. To be specific, when the receiver receives no light beam reflected by the retroreflector, the detector signals the presence of an object.
Such a detector is distinguished in emitting a narrow beam of light from the transmitter. Therefore, a light beam reflected by the retroreflector is directed to the receiver with certainty. False operation is avoided by not expanding the width of emitted and reflected light beams excessively.
The narrow beams of light, on the other hand, cause the detector to recognise a passing object which should not be detected. For example, a detector originally installed for detecting the passage of persons is operated by mistake, when a leave, insect or the like passes near the transmitter and interrupts a narrow beam of light.
Another detector suggested to solve this problem comprises two transmitters and receivers each. FIGS. 6 and 7 illustrate two types of detection units 100, 110 each of which comprises two transmitters 101, 101 and two receivers 102, 102. In the detection unit 100 of FIG. 6, transmitters 101, 101 are horizontally disposed on the upper part of a light emitting/receiving surface 103, and receivers 102, 102 are horizontally disposed on the lower part thereof. In contrast, in the detection unit 110 of FIG. 7, transmitters 101, 101 are vertically disposed on one side (on the right in the figure) of the light emitting/receiving surface 103, and receivers 102, 102 are vertically disposed on the other side thereof (on the left in the figure). Arrows in each figure indicate emitted and reflected beams of light.
Each of these detection units 100, 110 causes a detector to signal the presence of an object only when light beams emitted from the transmitters 101, 101 are both interrupted at the same time. In other words, the detector does not signal the presence of a small passing object which interrupts only either of the light beams, but it signals the presence or passage of an object when both beams of light are interrupted at the same time. False operation is avoided accordingly.
Nevertheless, the above detection units 100, 110 still have some problems. The detection unit 100 of FIG. 6, which applies a horizontal arrangement of the identical elements, is operated by mistake when an object 104 shown by an imaginary line in FIG. 6 passes near the light emitting/receiving surface 103 (e.g. when an object 104 whose longitudinal sides extend in the horizontal direction falls down), because the light beams emitted from both transmitters 101, 101 are interrupted at the same time. In such circumstances, although the light emitting/receiving surface 103 is covered only by half, the detector wrongly signals the presence of an object.
Likewise, the detection unit 110 of FIG. 7, which applies a vertical arrangement of the identical elements, is operated by mistake when an object 104 shown by an imaginary line in FIG. 7 passes near the light emitting/receiving surface 103 (e.g. when an object 104 whose longitudinal sides extend in the vertical direction crosses), because the light beams emitted from both transmitters 101, 101 are interrupted at the same time. In these circumstances, too, although the light emitting/receiving surface 103 is covered only by half, the detector wrongly signals the presence of an object.
Such false signalling can be avoided by disposing two detection units each comprising a transmitter and a receiver and spaced from each other by a predetermined distance. This arrangement is intended to prevent simultaneous interruption of the light beams emitted from both transmitters even when an object having the above-specified shape may fall or cross.
However, since this arrangement involves an additional step of disposing the two detection units at two separate locations, it increases the installation steps of the detector and raises the production cost.
Alternatively, the detection unit may be enlarged such that the optical elements can be disposed on the edges of a large light emitting/receiving surface with proper distances. False signalling may be prevented by separating the optical elements from each other.
Yet again, this arrangement is not a practical solution. This is because the large detection unit occupies a greater installation space, and also because of its poor appearance and higher production cost.
The present invention has been made in view of the above problems and intends to provide a retroreflective detector comprising a plurality of transmitting elements and receiving elements which can effectively prevent false signalling without increasing the size of the detector.
To achieve this object, the present invention presupposes that the retroreflective detector comprises a detection unit housing a plurality of transmitting elements and a plurality of receiving elements on a light emitting/receiving surface, and retroreflective means disposed opposite to the detection unit with a predetermined distance, the retroreflective detector determining the presence, passage or absence of an object in a space between the detection unit and the retroreflective means based on whether a beam of light emitted from each transmitting element is reflected by the retroreflective means and the reflected beam of light is received by each receiving element. In this retroreflective detector, the detection unit houses a plurality of pairs of a transmitting element and a receiving element in a matrix arrangement, such that every row and column of the matrix includes at least one transmitting element.
In this arrangement, every row and column of the matrix also includes at least one receiving element. Then, the transmitting elements and the receiving elements disposed in such matrix arrangement can define the maximum of widths both in the row direction and the column direction. When a relatively small object which need not be detected may fall or pass through a detection space, beams of light emitted from the transmitting elements are partially interrupted by the falling or passing object. The emitted light beams are interrupted completely only when a falling or passing object covers all of the transmitting elements. It is understood that all transmitting elements are covered by an object which is greater than the dimension of horizontally or vertically arranged transmitting elements. Likewise, in the case where reflected beams of light are completely interrupted by an object, all receiving elements are covered by an object which is greater than the dimension of horizontally or vertically arranged receiving elements. To summarise, the presence of an object is not recognised, unless the object present in or passing through the detection space is as great as or greater than the dimension of the light emitting/receiving surface. Consequently, the detector does not detect or signal the presence of an object by mistake, when the light emitting/receiving surface is only half-covered (see FIGS. 6 and 7).
Preferably, the above detector is modified to dispose two of the transmitting elements at the most distant positions from each other along a diagonal line based on rows and columns of the matrix, and to dispose two of the receiving elements at the most distant positions from each other along the other diagonal line.
This arrangement utilises the maximum area of the light emitting/receiving surface and separates one each of the transmitting elements and the receiving elements as far as possible from each of another transmitting element and receiving element. The presence or passage of an object is not recognised unless both of the relatively distant transmitters or both of the similarly distant receiving elements are covered at the same time. This arrangement further guarantees prevention of false signalling operation caused by a relatively small falling or passing object which need not be detected, without increasing the size of the whole detector.
In both arrangements, it is desirable that a horizontal distance between the transmitting elements which are most distant from each other in the horizontal direction and a horizontal distance between the receiving elements which are most distant from each other in the horizontal direction are smaller than a horizontal dimension of an object to be detected. It is also preferred that a vertical distance between the transmitting elements which are most distant from each other in the vertical direction and a vertical distance between the receiving elements which are most distant from each other in the vertical direction are smaller than a vertical dimension of an object to be detected.
These arrangements provide specific distances between the transmitting elements and between the receiving elements.
These arrangements further enhances the reliability of the detector by preventing failure to detect the presence or passage of an object in the space between the detection unit and the retroreflective means.